In plasma spray technology, modern systems often utilize a supersonic plasma jet operating in a low pressure enclosed environment, with powder being injected into the plasma jet such that minute molten droplets of metallic or ceramic material are caused to be coated with a highly adherent, temperature resistant layer. A particularly efficient example of such a system is provided in a co-pending application of Muehlberger et al entitled "System And Method For Plasma Coating", filed Nov. 26, 1979, Ser. No. 97,723 now U.S. Pat. No. 4,328,257 issued May 4, 1982 and assigned to the assignee of the present application. As described in that system, vacuum pumps coupled to an output conduit from the plasma spray chamber are arranged to draw and maintain a selected low pressure despite the supersonic flow of a high velocity ionized gas together with any shielding gases that may be used. The overspray that does not impinge on and adhere to the workpiece is drawn by the vacuum pump through an outlet conduit that includes a primary filter that eliminates particle matter in excess of a particular size, such as three microns. The generation of large quantities of this minute particulate is inherent in the plasma spraying process, but fundamentally incompatible with the operation of most high efficiency vacuum pumps. If the particulate is not removed from the outlet flow stream prior to entering the vacuum pump, its internal elements become coated and corroded, and in practice have failed catastrophically at an excessively high rate.
A simplistic solution to this problem is to utilize a filter, some types of which are referred to as "absolute filters," having extremely small openings and distorted flow paths, such that particulate of measurable size cannot penetrate. However, these filters interpose a significant pressure drop within the system, and in fact create problems of equal magnitude. The type of vacuum pump which will draw down to a sufficiently low pressure level cannot maintain the desired interior pressure in the system without being excessively large in size and using excessive amounts of power. A need therefore exists for an overspray filter and collector that introduces a low pressure drop into the system but that can efficiently remove substantially all particulate of below a specified size range.